OntoWiki – A Tool for Social, Semantic
Collaboration
Sören Auer1,2 , Sebastian Dietzold2 , and Thomas Riechert2
1
University of Pennsylvania, Department of Computer and Information Science
Philadelphia, PA 19104, USA,
[email protected]
2
Universität Leipzig, Institut für Informatik, Augustusplatz 10-11,
D-04109 Leipzig, Germany,
{lastname}@informatik.uni-leipzig.de
Abstract We present OntoWiki, a tool providing support for agile, distributed knowledge engineering scenarios. OntoWiki facilitates the visual
presentation of a knowledge base as an information map, with different
views on instance data. It enables intuitive authoring of semantic content, with an inline editing mode for editing RDF content, similar to
WYSIWYG for text documents. It fosters social collaboration aspects by
keeping track of changes, allowing to comment and discuss every single
part of a knowledge base, enabling to rate and measure the popularity
of content and honoring the activity of users. Ontowiki enhances the
browsing and retrieval by offering semantic enhanced search strategies.
All these techniques are applied with the ultimate goal of decreasing the
entrance barrier for projects and domain experts to collaborate using semantic technologies. In the spirit of the Web 2.0 OntoWiki implements
an ”architecture of participation” that allows users to add value to the
application as they use it. It is available as open-source software and a
demonstration platform can be accessed at http://3ba.se.
We present a tool supporting agile Knowledge Engineering in a pure Web environment. It is called OntoWiki since it is close in the spirit to existing Wiki
systems. Technologically however, the OntoWiki design is independent and complementary to conventional Wiki technologies. As such, the OntoWiki approach
differs from recently emerged strategies to integrate Wiki systems and the Semantic Web (cf. [6,5,8,11,12]). In these works it is proposed to integrate RDF
triples into Wiki texts in a special syntax. It is a straightforward combination of existing Wiki systems and the Semantic Web knowledge representation
paradigms. However, we see the following obstacles:
– Usability: The main advantage of Wiki systems is their unbeatable usability.
Adding more and more syntactic possibilities counteracts ease of use for
editors.
– Redundancy: To allow the answering of real-time queries to the knowledge
base statements have to be stored additionally in a triple store. This introduces a redundancy complicating the implementation.
– Scalability: Knowledge base changes which involve statements with different
subjects will scale very bad since all corresponding Wiki texts have to be
parsed and changed.
The OntoWiki strategy presented in this paper, on the contrary, does not
try to mix text editing with knowledge engineering, instead it applies the Wiki
paradigm of “making it easy to correct mistakes, rather than making it hard
to make them”[9] to collaborative knowledge engineering. The main goal of the
OntoWiki approach thus is to rapidly simplify the presentation and acquisition
of instance data from and for end users. This is achieved by regarding knowledge
bases as ”information maps”. Each node at the information map is represented
visually and intuitively for end users in a generic but configurable way and
interlinked to related digital resources. Users are further enabled to enhance the
knowledge schema incrementally as well as to contribute instance data agreeing
on it as easy as possible to provide more detailed descriptions and modelings.
Consequently, the following requirements have been determined for OntoWiki:
– Intuitive display and editing of instance data should be provided in generic
ways, yet enabling means for domains specific views.
– Semantic views allow the generation of different views and aggregations of
the knowledge base.
– Versioning and Evolution provides the opportunity to track, review and selectively roll-back changes.
– Semantic search facilitates easy-to-use full-text searches on all literal data,
search results can be filtered and sorted (using semantic relations).
– Community support enables discussions about small information chunks.
Users are encouraged to vote about distinct facts or prospective changes.
– Online statistics interactively measures the popularity of content and activity
of users.
– Semantic syndication supports the distribution of information and their integration into desktop applications.
In the remainder of the paper we propose strategies on how to put these
requirements into effect in a real-life system and report about implementation
in a prototypical OntoWiki on the basis of Powl [1], a framework for Semantic
Web application development. To stress the generic character of OntoWiki, the
figures in this paper show screenshots of the OntoWiki prototype with a knowledge base collaboratively developed3 and containing information about scientific
conferences, as well as another publicly available knowledge base4 containing
information about research projects, people and publications at a research institute.
3
4
at http://3ba.se
http://www.aifb.uni-karlsruhe.de/viewAIFB OWL.owl
1
Visual Representation of Semantic Content
The compromise of, on the one hand, providing a generic user interface for
arbitrary RDF knowledge bases and, on the other hand, aiming at being as
intuitive as possible is tackled by regarding knowledge bases as ”information
maps”. Each node at the information map, i.e. RDF resource, is represented as
a Web accessible page and interlinked to related digital resources. These Web
pages representing nodes in the information map are divided into three parts: a
left sidebar, a main content section and a right sidebar. The left sidebar offers
the selection of content to display in the main content section. Selection opportunities include the set of available knowledge bases, a class hierarchy browser
and a full-text search. Once a selection is made, the main content section will
arrange matching content in a list view linking to individual views for individual
instances (cf. 1). The right sidebar offers tools and complementary information
specific to the selected content.
Figure 1. List view (left) and view of an individual instance with expanded
inline reference view (right).
List views allow to view a selection of several instances in a combined view. The
selection of instances to display can be either based on class membership or on
the result of a selection by facet or full-text search. OntoWiki identifies those
properties used in conjunction with the instances of the selection. The display
of the corresponding property values for each instance can be switched on, thus
resulting in a tabular view. Furthermore, each individual instance displayed is
linked to an individual view of that instance.
Individual views combine all the properties attached to an particular instance.
Property values pointing to other individuals are rendered as HTML links to
the corresponding individual view. Alternatively, to get information about the
referenced individual without having to load the complete individual view it
is possible to expand a short summary (loaded per AJAX) right where the
reference is shown. The right sidebar provides additionally information about
similar instances (of the same type) and incoming links (i.e. references from
other instances).
Different Views on Instance Data The OntoWiki prototype facilitates different views on instance data. Such views can be either domain specific or generic.
Domain specific views have to be implemented as plug-ins. Generic views provide
visual representations of instance data according to certain property values. The
following views are currently implemented:
Figure 2. Map view (left) and calendar view (right) of instance data.
Map View. If the selected data (either a single instance or a list of instances)
contains property values representing geographical information (i.e. longitude
and latitude coordinates) a map view provides information about the geographical location of the selected data (cf. Figure 2). Technically, this view is realized
by integrating the Google Maps API5 . However, the integration is bi-directional,
since objects displayed in the map can be expanded and instance details are dynamically fetched from the knowledge base and displayed directly within the
map view. The selection of instances to be displayed can be furthermore the
result of a facet-based filtering (cf. Section 4).
Calendar View. Instances having property values with the associated datatype
xsd:date can be displayed in a calendar view (cf. Figure 2). As for the map
view the selection of instances displayed in the calendar view can be the result
of a facet-based filtering. Each item displayed is linked to the individual view of
5
http://www.google.com/apis/maps/
the corresponding instance. The sidebar offers a link to export calendar items in
iCal format, which enables to import the selected calendar items into a desktop
calender application.
2
Collaborative Authoring
To enable users to edit information presented by the OntoWiki system as intuitively as possible, the OntoWiki approach supports two complementary edit
strategies for the knowledge base:
– Inline editing, the smallest possible information chunks (i.e. statements) presented in the OntoWiki user interface are editable for users.
– View editing, common combinations of information (such as an instance of
a distinct class) are editable in one single step.
Both editing strategies are supported by a mixed client and server side concept
identification and reuse technology and a library of generic editing widgets. In the
remainder of this section the editing strategies and their supporting technologies
are presented in more detail.
2.1
Inline Editing
For human users it is important that the statements of a knowledge base are
presented on the user interface in a way facilitating the efficient reception of this
information. To achieve this goal information should be ordered and grouped
and as a result of this information appearing redundant should be omitted. If
the context clarifies, for example, that some information describes a distinct
concept (e.g. since the OntoWiki page for a person was accessed) the concept
will be displayed only once on the OntoWiki user interface, even though all
the statements describing the concept contain the concepts URI reference as
subject. If furthermore a property (e.g. referencing publications) occurs multiple
times (e.g. since the person described is author of multiple publications) those
statements should be grouped together and the label of the property should be
displayed only once (cf. Figure 3).
Even though such a human friendly representation of the statements contained in the knowledge bases conceals the original statement structure the
OntoWiki system is aware which information displayed on the user interface
originated from what statements. To enable users to rapidly edit or add statements as soon as they notice mistakes or missing information OntoWiki features
an inline editing mode. This means that all information originating from statements presented on the OntoWiki user interface is equipped with a small edit
button as well as an add button (cf. Figure 3). After clicking one of those buttons a resource editor (cf. Figure 4) is loaded and the corresponding statement
can be easily edited or a similar content (i.e. a statement with same subject and
predicate) can be added.
Figure 3. OntoWiki instance display with statement edit buttons (left). Statement editor with interactive search for predefined individuals based on AJAX
technology (right).
This strategy can be seen analogous to the WYSIWYG (What You See Is
What You Get) editing strategy for text editing, since information can be edited
in the same environment as it is presented to users.
2.2
Concept Identification and Reuse
Knowledge bases become increasingly advantageous, if once defined concepts
(e.g. classes, properties, or instances) are as much reused and interlinked as
possible. This especially eases the task of rearranging, extracting and aggregating
knowledge. To become part of the daily routine for even inexperienced and rare
users of the OntoWiki system already defined concepts should be suggested to
the user, whenever he is requested to contribute new information. In a Web
based environment and for highly scalable knowledge bases conventional Web
technologies were the major obstacles for this.
Conventional Web technologies do not support large data sets to be handled
at the client (browser) side. But this is usually needed when working with large
knowledge bases. To overcome this limitation, reloading of web pages becomes
necessary. This approach is time consuming and requires multiple user interactions. Recently, with the deployment of more sophisticated Web browsers, supporting modern JavaScript and XML technologies, mixed server and client side
web applications became possible. These were recently named AJAX (Asynchronous JavaScript and XML) and early adopters such as Google-Maps6 or
Flickr7 make extensive use of them.
6
7
http://maps.google.com
http://www.flickr.com
The OntoWiki uses the AJAX technology to interactively propose already
defined concepts while the user types in new information to be added to the
knowledge base (cf. Figure 3). To realize this interactive search, all URI references and literals are indexed for full-text searches in the statement repository.
2.3
Editing Widgets
For convenient editing of differently typed literal data the OntoWiki system
provides a library of reusable user interface components for data editing, called
widgets. Such widgets are implemented in a server side programming language
(e.g. PHP), they generate HTML fragments together with appropriate Cascading
Style Sheet definitions and optionally JavaScript code. They may be customized
for usage in specific contexts by widget configurations. The following widgets
are currently provided by the prototypical OntoWiki implementation:
–
–
–
–
–
–
Statements: allows editing of subject, predicate, and object.
Nodes: edit literals or resources.
Resources: select and search from/for existing resources
Literals: literal data in conjunction with a data type or a language identifier.
Widgets for specific literal data types: e.g. dates, HTML fragments.
File widget: allows uploading of files to the OntoWiki system.
All widgets can be configured. The OntoWiki system allows to define and
attach certain sets of configurations to a specific widget. In addition to widget
specific configurations, generic widget configuration which should be applicable
to all widgets includes HTML attributes such as class, height and width of the
widget, or arbitrary CSS styles.
A widget selection connects a widget configuration with a context. Contexts
are the data type of the literal to be edited, the property of the statement which’s
object is edited, the property in conjunction with a specific class, the knowledge
base the node to be edited belongs to, as well as the editing user and her group.
2.4
View Editing
Editable views are combinations of widgets to edit a specific view on the knowledge base in one single step. The OntoWiki system provides the following types
of editable views:
– Metadata: comments, labels, and annotations (such as versioning and compatibility information) which can be attached to arbitrary resources are combined in a metadata view.
– Instances: An instance view combines all properties attached to the instance’s class or one of the super-classes. For large knowledge bases this
might include a large amount of properties. The OntoWiki system thus allows to restrict the view to such properties which are really used in conjunction with other instances of the same class. On the basis of range definitions
Figure 4. Editing of a property’s values at many instances at once (left). Dynamically generated form combining different widgets based on an OWL class
definition (right).
for the property, OntoWiki selects appropriate editing widgets. Additional
properties can be added on-the-fly, the system will ask the user in a next
step to specify the property’s characteristics (e.g. domain, range, cardinality
restrictions).
– Views: The earlier described inline-editing technique allows to edit arbitrary views. The columns of list views arranging many instances in a tabular
way for example can be easily edited at once, thus allowing to rapidly add
”horizontal” knowledge (across several instances) to the knowledge base (cf.
Figure 4).
3
Enabling Social Collaboration
A major aim of OntoWiki is to foster and employ social interactions for the
development of knowledge bases. This eases the structured exchange of metainformation about the knowledge base drastically and promotes collaboration
scenarios where face-to-face communication is hard. Making means of social
interactions as easy as possible furthermore contributes in creating an ”architecture of participation” that allows users to add value to the system as they use
it. Social collaboration within OntoWiki is in particular supported by:
Change tracking. All changes applied to a knowledge base are tracked. OntoWiki
enables the review of changes on different levels of detail (see also [3]) and optionally restricted to a specific context, such as changes on a specific instance,
changes on instances of a class, or changes made by a distinct user. In addition
to present such change sets on the Web, users can subscribe to get information
about the most recent changes on objects of their interest by email or RSS/Atom
feeds.
Commenting. All statements presented to the user by the OntoWiki system may
be annotated, commented, and their usefulness can be rated. This enables community driven discussions, for example about the validity of certain statements.
Technically, this is implemented on the basis of RDF reifications, which allow to
make statements about statements. Small icons attached to an object of a statement within the OntoWiki user interface indicate that such reifications exist (cf.
Figure 5). Positioning the mouse pointer on such an icon will immediately show
up a tool tip with the most recent annotations; clicking on the icon will display
them all.
Figure 5. Comments attached to statements.
Rating. OntoWiki allows to rate instances. Users have to be registered and
logged into the system to participate in order to avoid duplicate ratings by
the same user. However, a user may change his rating for a certain instance.
Special annotation properties allow the creation of rating categories with respect
to a certain class. Instances of the class can then be rated according to these
categories, thus allowing for example the rating of instances of a class publication
according to categories originality, quality and presentation.
Popularity. All accesses to the knowledge base are logged thus allowing to arrange views on the content based on popularity. As with ratings or user activity,
the popularity of content can be measured with respect to a certain knowledge
base or fragment of it (e.g. popularity with respect to class membership). This
enables users to add value to the system as they use it.
Activity/Provenance. The system keeps record of what was contributed by
whom. This includes contributions to the ontology schema, additions of instance
data or commenting. This information can be used to honor active users in the
context of the overall system, a specific knowledge base or a fragment of it (e.g.
instance additions to some class). This way it contributes to instantly gratify
users for their efforts and helps building a community related to certain semantic content.
4
Semantic Search
To realize the full potential of a semantic browsing experience the semantic
structuring and representation of content should be employed to enhance the
retrieval of information for human users. OntoWiki implements two complementary strategies to achieve this goal.
4.1
Facet-based Browsing
Taxonomic structures give users exactly one way to access the information. Furthermore, the development of appropriate taxonomic structures (whether e.g.
class or SKOS keyword hierarchies) requires significant initial efforts. As a payas-you-go strategy, facet-based browsing allows to reduce the efforts for a priori
knowledge structuring, while still offering efficient means to retrieve information. Facet-based browsing was also implemented by the Longwell Browser8 for
RDF data and it is widely deployed in the shape of tagging systems of the Web
2.0 folksonomies. To enable users to select objects according to certain facets,
all property values (facets) of a set of selected instances are analyzed. If for a
certain property the instances have only a limited set of values, those values are
offered to restrict the instance selection further. Hence, this way of navigation
through data will never lead to empty results. The analyzing of property values
though can be very resource demanding. To still enable fast response times the
OntoWiki system caches the results of of a property value analysis for later reuse
and invalidates those cache objects selectively if values of the respective property
are updated (see [2, Chapter 5] for details).
4.2
Semantically Enhanced Full-text Search
OntoWiki provides a full-text search for one or multiple keywords occurring in
literal property values. Since there can be several property values of a single
individual containing the search string the results are grouped by instances.
They are ordered by frequency of occurrence of the search string. Search results
may be filtered to contain only individuals which are instances of a distinct class
or which are described by the literal only in conjunction with a distinct property
(cf. Figure 6).
A semantic search has significant advantages compared to conventional fulltext searches. By detecting classes and properties, contain matching instances,
the semantic search delivers important feedback to the user how the search may
be successfully refined.
The semantic search is currently implemented as a search in the local RDF
store. In conjunction with a crawler, which searches, downloads, and stores arbitrary RDF documents from the web, OntoWiki can be easily transformed in
a Semantic Web search engine.
8
http://simile.mit.edu/longwell/
Figure 6. User interface for the semantic search in the OntoWiki system. After
a search for ”York” it suggested to refine his search to instances with one of the
properties swrc:address, swrc:booktitle or swrc:name.
5
Implementation and Status
OntoWiki is implemented as an alternative user interface to the schema editor
integrated in Powl. Powl is a platform for Semantic Web application development
realized in a 3-tier architecture consisting of storage tier, object-oriented API
and user interfaces (cf. Figure 7). Many of the requirements for OntoWiki were
gathered from use cases of Powl.
OntoWiki was implemented in the scripting language PHP, thus allowing to
be easily deployed on most Web hosting environments. The application is available as open-source software from SourceForge9 . A publicly available knowledge
repository on the basis of OntoWiki is available at http://3ba.se.
The system is designed to work with knowledge bases of arbitrary size (only
limited by disk space). This is achieved by loading only those parts of the knowledge base into main memory which are required to display the information requested by the user on the screen (i.e. to render a Web page containing this
information).
Currently, OntoWiki is extended and adopted within a variety of R&D projects.
The project SoftWiki10 for example is developing a prototype based on OntoWiki, which aims to employ OntoWiki’s social collaboration functionality for
end-user driven Requirements Engineering of massively distributed software development projects. For the project Orchestra [7] OntoWiki’s storage, browsing
and retrieval functionality is envisioned to be used as a shared repository for
9
10
http://powl.sf.net
http://softwiki.de
Figure 7. Architecture of Powl and OntoWiki.
ontologies and queries in the bio-informatics domain. In the project ”Vernetzte Kirche” [4] Powl and parts of OntoWiki were applied to foster a meta-data
initiative for social, cultural and religious content.
6
Conclusion
In this paper we presented the OntoWiki approach, exemplary exhibiting how
tool support for agile, collaborative knowledge engineering scenarios can be provided. Since the OntoWiki system is based technologically on Powl, we stressed
in this paper especially aspects facilitating the usage of the OntoWiki system.
These include the visual presentation of a knowledge base as an information
map, social collaboration aspects as well as a semantic search strategy. Such
efforts, which decrease the entrance barrier for domain experts to collaborate
using semantic technologies, are in particular crucial to gain a maximized impact on collaborative knowledge engineering. Examples from other domains, such
as Community Content Management and Software Development, showed that
such efforts can have an enormous impact on distributed collaboration, thus enabling completely new products and services. Conventional Wiki technologies
for example radically simplified the editing process and enabled the Wikipedia
project11 to attract a multitude of editors finally succeeding in the creation of
the worlds largest encyclopedia. Technologies for distributed collaborative software development as CVS and Subversion12 or the SourceForge13 platform made
it possible to develop almost any standard software for private or business needs
11
12
13
http://wikipedia.org
http://subversion.tigris.org
http://sf.net
largely in absence of strong, centralized, commercial corporations. The aim of
OntoWiki is to contribute giving the Semantic Web a much broader basis.
Application domain. The OntoWiki system is technologically independent of and
complementary to conventional text Wiki systems. It enables the easy creation
of highly structured content by distributed communities. The following points
summarize some limitations and weaknesses of OntoWiki and thus characterize
the application domain:
– Environment: OntoWiki is a Web application and presumes all collaborators
working in a Web environment, possibly spatially distributed.
– Usage Scenario: OntoWiki focuses on knowledge engineering projects where
a single, precise usage scenario is either initially not (yet) known or not
(easily) definable.
– Reasoning: Application of reasoning services is (initially) not mission critical.
– Budget: Only a small amount of financial and personnel resources are available.
Open issues and potential future work.
– Implement a privilege system and access control for and on the basis of the
RDF data model with support for rights management on higher conceptual
levels than that of statements.
– Obtain more case studies, in particular independent comparisons, are needed
to provide further evidence to see whether OntoWiki lives up to its promises.
– Examine possibilities to tighter integrate the Description Logic reasoning
services into OntoWiki.
– Establish better methods of interaction with existing content and knowledge
management systems.
Further related work. In addition to the affinity with Wiki systems and Web portals in general the OntoWiki approach can be seen as a representative of a new
class of semantic portals (cf. [13]). The SEAL SEmantic portAL [10] for example
exploits semantics for providing and accessing information at a portal as well
as constructing and maintaining the portal. Due being based on a rather static
methodology [14] it focuses less on spontaneous, incremental enhancements of
the knowledge base than OntoWiki. Another approach to develop a semantic
portal is the website of the Mindswap project14 . Semantic Web knowledge representation standards are used as primary data source and for interoperability,
the editing and publishing process as well as collaboration aspects however seem
to be either not tackled or publicised.
Acknowledgments
This research was supported in part by the following grants: BMBF (SE2006
#01ISF02B), NSF (CAREER #IIS-0477972 and SEIII #IIS-0513778).
14
http://www.mindswap.org/first.shtml
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